Existing musical instruments are divided into three categories: strings, percussion, and wind. Strings are essentially one dimensional solids (i.e. they are long and thin, having a relatively small cross section). Percussion is typically a two-dimensional (i.e. flat and relatively thin) or three-dimensional (bulk) solid. Wind instruments run on matter in its gaseous state.
More generally, various researchers have categorized all known musical instruments into five categories: idiophones, membranophones, chordophones, aerophones, and electrophones. This categorization scheme was devised to categorize all possible musical instruments either known or to be made in the future. This system originated thousands of years ago, was adopted by Victor-Charles Mahillon, and then further refined by Hornbostel and Sachs, and is often referred to as the Hornbostel Sachs Musical Instrument Classification Scheme.
The first three categories refer to solid matter, in three, two, and one dimension, i.e. idiophones make sound from bulk (3d) solid matter. Membranophones make sound from membranes (flat thin, essentially 2 dimensional solid matter). Chordophones make sound from stings which are essentially one dimensional solid matter.
Instruments like the piano problematize the “strings-percussion-wind” taxonomy because the piano is both a string instrument and a percussion instrument. This has led other experts such as Andre Schaeffner to classify acoustic instruments into two large categories: solid and gas. The first category, category “I”, makes sound by matter in its solid state. The second category, category II, makes sound by matter in its gaseous state.
Musical instruments can also be electrically amplified, and remain in the same category despite this amplification.
Additionally, a category of electrophones refers to instruments in which the sound does not originate from the material world, and is instead originated electrically.
Another state-of-matter, namely liquid, has been found relevance in musical instruments. For example, the ancient Greeks and Romans used water as a supply of power, in order to blow air into organ pipes. These ancient instruments like the “water organ” or “hydraulis” used water as a power source, or as a means to store energy, which was then used to push wind through organ pipes.
In a similar way, modern church organs are examples of water organs because they use hydroelectricity (electricity that is generated by a waterfall) as a source of power to run the electric motor that powers the blower, which blows the wind (air) into the pipes to make the sound.
Sounds can also be produced underwater. For example, municipal swimming baths, various public and private pools, and the like, often have underwater loudspeakers so that music can be played for people to hear underwater. This also facilitates safety, so that announcements over the Public Address (PA) system can be heard underwater.
Some animals such as dolphins and porpoises can make sounds underwater. They do this by having air pockets in which they make sound in air, which then is audible underwater.